From the middle the frame will spread of course to the two extremities and light the other two matches, probably this side first where the two phosphorous heads meet. [Illustration] Well, nothing of the sort happens. When the volume of the burning match has diminished, and consequently its rigidity also, the force of its resistance grows weaker as the combustion proceeds. A moment comes when the two vertical matches, trying to assume again their original position, throw off, with a sway, the burning horizontal match. The burning match was rendered flexible in the middle, and is not at all burned at the ends, and the two matches remain standing as before. To Make an Exact Balance. To construct by yourselves, with the help of simple materials a balance of great precision may seem impossible. Nevertheless it can be done. A ruler, a tin box, (in which blacking was contained, for example) three blocks of wood, two pins, thread, four nails, a small piece of glass, and cardboard are all the necessary materials, and now to work. At a short distance from the center of the ruler, and on a cross line with one another, stick two pins so that they come out a little on the other side. At one end of the ruler, in C, nail a small piece of your box. [Illustration] At the spot, where the hook to which the scale is suspended, is to hang, make an indentation with the point of a nail, so that the hook does not shift at the other extremity, in A, fasten a flat piece of tin, which will form one of the scales of your balance. At the end of this pan solder a pin point downwards. Your second scale, B, destined to contain the object or substances to be weighed, will be formed by the lid of the blacking-tin. On its rim at nearly equal distances pierce four holes, on which the suspension-strings will be tied, the latter at their upper end being united together in one string, which is tied to a hook (a bent pin or fishing hook will do.) Now the point of support remains to be constructed. On a wooden square, rather thick, E, fix another block, G, on which gum a piece of glass. In the largest block knock four nails to prevent the shaft of the balance swerving from right to left. The small truncated pyramid, D, which you perceive on the left of the design, and which is graduated, serves as bench-mark. In order to weigh you use the method due to Borda, called the method of double weights. Place in the scale A a weight which you think is slightly over the one of the substance or object to be weighed. Then the scale B being occupied, get equilibrium by shifting more or less towards the ruler, the weight on the scale A. Then note the division indicated by the pin point, and take from scale B the article placed there, and put therein weights until the point of scale A tells you that the equilibrium is the same as when the substance was in the scale. It is not necessary that this balance be exact, provided it answers the very small differences in the pans.